The present invention relates to a magneto-resistive device, and a magnetic head and a head suspension assembly using the same.
With the trend to a larger capacity and a smaller size of hard disk drives (HDD), heads are required to have a higher sensitivity and larger output. To meet this requirement, strenuous efforts have been made to improve characteristics of GMR head (Giant Magneto-Resistive Head) currently available on the market. On the other hand, intense development is under way for a tunnel magneto-resistive head (TMR head) which can be expected to have a resistance changing ratio twice or more higher than the GMR head.
Generally, the GMR head differs from the TMR head in the head structure due to a difference in a direction in which a sense current is fed. A head structure adapted to feed a sense current in parallel with a film plane, as in a general GMR head, is referred to as a CIP (Current In Plane) structure, while a head structure adapted to feed a sense current perpendicularly to a film plane, as in the TMR head, is referred to as a CPP (Current Perpendicular to Plane) structure. Since the CPP structure can use a magnetic shield itself as an electrode, it is essentially free from short-circuiting between the magnetic shield and a device (defective insulation) which is a serious problem in reducing a lead gap in the CIP structure. For this reason, the CPP structure is significantly advantageous in providing a higher recording density.
Other than the TMR head, also known as a head in CPP structure is, for example, a CPP-GMR head which has the CCP structure, though a spin valve film (including a specular type multilayer film and a dual spin valve type magnetic multilayer film) is used for a magneto-resistive device.
Any type of CPP-based head has an upper electrode and a lower electrode for supplying a current to a magneto-resistive layer formed on a base, formed on the top (opposite to the base) and on the bottom (close to the base) of the magneto-resistive layer, respectively. Generally, for reasons of manufacturing process, the base formed with the magneto-resistive layer is left in the atmosphere after the magneto-resistive layer is formed and before the upper electrode is formed. In this event, for preventing the top surface of the magneto-resistive layer from being oxidized in the air to damage the characteristics of the magneto-resistive layer such as an MR ratio, a non-magnetic metal layer, called a “cap layer,” is previously formed as a protection film on the top surface of the magneto-resistive layer. Ta or the like is used for the non-magnetic metal layer. Then, in the CPP-based head, the upper electrode is electrically connected to the magneto-resistive layer through the non-magnetic metal layer.
In the CCP-based head, the magneto-resistive layer is supplied with a current through the upper electrode and non-magnetic metal layer, so that a good electrical contact must be maintained between the upper electrode and non-magnetic metal layer to reduce the resistance. However, since Ta or the like is used for the non-magnetic metal layer, the surface of the non-magnetic metal layer is oxidized in the air while the base formed with the magneto-resistive layer and non-magnetic metal layer is left in the atmosphere. Thus, if another layer such as an upper electrode is formed on the oxidized non-magnetic metal layer, a good electrical contact cannot be maintained between the upper electrode and non-magnetic metal layer. To solve this inconvenience, the surface oxide film is removed from the non-magnetic metal layer by dry etching (including a whole dry process such as sputter etching, ion beam etching, and the like) within the same vacuum chamber in which the upper electrode and the like are deposited, prior to the formation of another layer such as the upper electrode on the non-magnetic metal layer.
It is a conventional technical common sense that the thickness of the non-magnetic metal layer should be reduced as much as possible to such an extent that the oxidation on the surface of the magneto-resistive layer can be effectively prevented in the air, so that the thickness of the non-magnetic metal layer has been set to as thin as approximately 5 nm. This is because it is believed that a thinner non-magnetic metal layer results in a better end face shape of a magneto-resistive layer, which is determined when the magneto-resistive layer is milled into a desired shape, to improve the characteristics of the magneto-resistive device.
However, the result of a research made by the inventors revealed that the foregoing technical common sense is not always correct. Specifically, factors conventionally taken into consideration are the prevention of oxidized surface of the magneto-resistive layer, and the end face shape of the magneto-resistive layer, while overlooking a damage to the magneto-resistive layer by an ion beam in a process of removing a surface oxide film from the non-magnetic metal layer. For example, it has been revealed that, in a TMR head, when a tunnel barrier layer is extremely reduced in thickness (for example, to 1 nm or less) in order to reduce the resistance of a magneto-resistive layer itself, the tunnel barrier layer is largely damaged by the ion beam in the process of removing a surface oxide film from a non-magnetic metal layer, resulting in an extremely reduced MR ratio and an occasional failure in utilization as a magnetic head. While such a damage caused by the ion beam is particularly large in the TMR head, the same is true in other CPP-based heads such as a CPP-GMR head.